Interstitial fluid pressure is one of the main forces regulating the movement of fluid across the pulmonary capillary membrane and through the lung interstitium. We have recently employed a variety of techniques, wick-catherter, open-ended needles, and 2-5mu micropipets to measure directly the interstitial fluid pressure around large arteries, bronchi and veins in isolated dog lung lobes, as a function of transpulmonary pressure, vascular pressure and extravascular water accumulation. These measurements agreed with our estimates of perivascular surface pressures from a continuum mechanics analysis suggesting that surface pressures in the form of nonuniform parenchymal stresses in the adjoining parenchymal tissue attachments is the major determinant of the peribronshovascular interstitial fluid pressure. Interstitial fluid pressures as a function of water accumulation at equilibrium suggested that alveolar surface tension is the limiting factor determining interstitial fluid pressure during alveolar edema formation. We plan to use similar analytical and experimental techniques to explore further the nonuniformity of interstitial fluid pressure in the peribronchovascular interstitium. First, we shall do a continuum analysis of the bronchial-arterial interaction to determine the distribution of surface stress. Second, we shall attempt to verify the stress distribution by direct measurements of interstitial fluid pressure at different sites in the peribronchovascular interstitium in isolated dog lobes. Third, we shall measure bronchial and vascular calibers and perivascular interstitial fluid pressure as a function of water accumulation and lobe height. These experiments will establish whether there is a gravity dependent gradient in perivascular interstitial fluid pressure. Finally, we shall measure in addition to perivascular interstitial fluid pressure alveolar liquid (subphase) pressures as a function of extravascular water accumulation and lung transpulmonary pressure to verify directly that alveolar surface tension reduces perivascular interstitial fluid pressure below the alveolar pressure during alveolar edema.